1. Field of the Invention
The invention relates generally to lensed fibers for delivering and collecting light in optical communication networks. More specifically, the invention relates to a tapered lensed fiber for focusing and condenser applications and a method for forming the tapered lensed fiber.
2. Background Art
A lensed fiber is a monolithic device having an optical fiber terminated with a lens. Lensed fibers are advantageous because they are easy to assemble, i.e., they do not require active fiber-lens alignment and gluing of fiber to lens, they have low insertion loss, and they enable component miniaturization because they can be made very small. The coefficient of thermal expansion of the lens can be matched to that of the optical fiber to achieve better performance over a temperature range. Lensed fibers are easily arrayed and are therefore desirable for making arrayed devices, for use in silicon optical bench applications, for aligning optical fibers to planar waveguides, and so forth. In addition, the spot size and working distance of the lensed fiber can be tailored for a specific application. For example, the spot size and working distance can be tailored to produce smaller beam diameters that can allow use of smaller micro-electro-mechanical-systems (MEMS) mirrors in optical switches.
There are three main types of lensed fibers, including collimating lensed fibers, focusing lensed fibers, and tapered lensed fibers. FIG. 1 shows a prior-art collimating lensed fiber 20 having a planoconvex lens 22 fusion-spliced to one end of an optical fiber 24. The optical fiber 24 may be a single-mode or multimode fiber. In the illustration, the optical fiber 24 is a single-mode fiber. The planoconvex lens 22 is formed from a coreless fiber, and the front surface of the lens 22 is shaped like a sphere. The spherical surface of the lens 22 is typically formed by melting the coreless fiber using an electric arc or a laser beam. The spherical surface of the lens 22 acts as a collimator, expanding the light coming out of the optical fiber 24 into a collimated beam. In practice, the lensed fiber 20 is used to couple light from one fiber to another.
FIG. 2A shows a prior-art focusing lensed fiber 26 having a first square-law index fiber 28 fusion-spliced to one end of a single-mode fiber 30. A second square-law index fiber 32 is connected to the first square-law fiber 28. A convex surface 34, which acts as a lens, may be provided at a distal end 36 of the second square-law index fiber 32. In general, the radius of curvature of the convex surface 34 is smaller than the radius of curvature of the lens for the collimating lensed fiber (see lens 22 in FIG. 1). The focusing lensed fiber 26 focuses light into a spot and is useful for focusing applications, such as focusing a beam onto a detector or receiver and coupling light from a laser source to an optical fiber.
There are various methods for providing the convex surface 34 at the distal end 36 of the second square-law index fiber 32. One method involves melting the distal end 36 to form a hemispherical surface. A second method involves chemically etching the distal end 36 to form the convex surface 34. A third method, which is illustrated in FIG. 2B, involves fusion-splicing a coreless fiber 38 to the distal end 36 of the second square-law index fiber 32 and then melting the coreless fiber 38 to form a hemispherical surface. Melting is usually based on electric discharge.
FIG. 3 shows a prior-art tapered lensed fiber 40 having a taper 42 formed at a tip of a single-mode fiber 44. The taper 42 has a convex surface 46 that acts as a lens. The taper 42 can be achieved by grinding and/or polishing the tip of the single-mode fiber 44. The tapered lensed fiber collimates light over a short working distance. The tapered lensed fiber can be used for coupling light between an optical fiber and a laser source or an optical amplifier or a planar waveguide.
In one aspect, the invention relates to a tapered lensed fiber which comprises a tapered multimode fiber having a gradient-index core and an optical fiber attached to the tapered multimode fiber.
In another aspect, the invention relates to a method for forming a tapered lensed fiber which comprises attaching an optical fiber to a multimode fiber having a gradient-index core, applying heat to a surface of the multimode fiber, and pulling the multimode fiber into a taper.
In another aspect, the invention relates to a method for forming a tapered polarization-maintaining fiber which comprises attaching a polarization-maintaining fiber to a multimode fiber having a gradient-index core, applying heat to a surface of the multimode fiber, and pulling the multimode fiber into a taper.
In another aspect, the invention relates to a tapered polarization-maintaining fiber which comprises a tapered multimode fiber having a gradient-index core and a polarization-maintaining fiber attached to the tapered multimode fiber.
Other features and advantages of the invention will be apparent from the following description and the appended claims.